JIS G3101 SS400 vs JIS G3106 SM400: Steel Standard Comparison for Export
Japanese Industrial Standards (JIS) govern a significant portion of carbon steel exports from China to Southeast Asia, the Middle East, and automotive supply chains. Among the most commonly specified grades, SS400 (JIS G3101) and SM400 (JIS G3106) appear frequently in procurement documents — yet buyers often misunderstand their distinct purposes and mechanical requirements.
This article provides a technical comparison of these two standards, clarifying when each applies and what quality evidence buyers should demand.
1. Standard Scope and Purpose
| Standard | JIS G3101 | JIS G3106 |
|---|---|---|
| Title | Rolled steels for general structure | Rolled steels for welded structure |
| Primary Use | General construction, machinery, bridges | Welded structural components, ships, pressure vessels |
| Weldability Focus | Standard | Enhanced (carbon equivalent controls) |
| Impact Test | Not required | Required for SM400B/C at 0°C / -20°C |
Core distinction: G3101 addresses general-purpose structural steel. G3106 specifically targets applications where welding is the primary joining method, imposing stricter chemical controls to prevent cold cracking.
2. Grade Designation System
SS400 has a single grade designation indicating minimum tensile strength of 400 N/mm². SM400 subdivides into three quality levels:
- SM400A: Basic grade, impact test not required
- SM400B: 0°C Charpy V-notch impact test required (≥ 27J)
- SM400C: -20°C Charpy V-notch impact test required (≥ 27J)
The A/B/C suffix directly correlates to toughness requirements and price. SM400C commands approximately 8 – 12% premium over SM400A in China export markets.
3. Chemical Composition Limits
| Element | SS400 | SM400A | SM400B | SM400C |
|---|---|---|---|---|
| C (max) | — (by agreement) | ≤ 0.23% | ≤ 0.20% | ≤ 0.18% |
| Si | — | ≤ 0.35% | 0.15 – 0.35% | 0.15 – 0.35% |
| Mn | — | ≥ 0.60% | 0.60 – 1.20% | 0.60 – 1.20% |
| P (max) | ≤ 0.050% | ≤ 0.035% | ≤ 0.035% | ≤ 0.035% |
| S (max) | ≤ 0.050% | ≤ 0.035% | ≤ 0.035% | ≤ 0.035% |
| Ceq (max) | Not specified | ≤ 0.36% | ≤ 0.36% | ≤ 0.36% |
Note: SS400 places no mandatory carbon limit — suppliers and buyers agree on actual values. This flexibility makes SS400 cheaper but less predictable for welding.
4. Mechanical Properties Comparison
| Property | SS400 | SM400 (all grades) |
|---|---|---|
| Yield Strength (t ≤ 16mm) | ≥ 245 MPa | ≥ 245 MPa |
| Tensile Strength | 400 – 510 MPa | 400 – 510 MPa |
| Elongation (t ≤ 16mm) | ≥ 21% | ≥ 18% |
| Bend Test (180°) | t ≤ 16mm: d = 2a | t ≤ 16mm: d = 2a |
| Impact Test | None | B: 0°C / C: -20°C, ≥ 27J |
5. Welding Performance
SM400’s controlled carbon equivalent (Ceq ≤ 0.36%) ensures predictable weldability without preheating for sections up to 25mm thickness at ambient temperatures above 5°C. SS400 may require preheating depending on actual carbon content, which varies by supplier.
Welding recommendation summary:
- SM400B/C: No preheat required for t ≤ 25mm; use basic-coated electrodes (E7016/E7018)
- SS400: Verify actual Ceq from MTC; preheat to 50 – 100°C if Ceq > 0.38%
- Both grades: Avoid high heat input (> 50 kJ/cm) to prevent coarse grain formation in HAZ
6. Equivalent Standards by Country
| Standard | SS400 Equivalent | SM400 Equivalent |
|---|---|---|
| GB/T 700 (China) | Q235B | Q235C / Q235D |
| ASTM A36 (USA) | A36 (close) | A283 Gr. C / D |
| DIN EN 10025 (EU) | S235JR | S235J0 / S235J2 |
| ISO 630 | E235B | E235C / E235D |
7. Procurement Checklist for Chinese Exports
When ordering JIS-certified structural steel from Chinese mills:
- Specify exact standard + grade: “JIS G3106 SM400B” not just “SM400”
- Demand impact test certificates for SM400B/C — verify test temperature and actual absorbed energy
- Check carbon equivalent calculation using IIW formula: Ceq = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15
- Verify plate thickness capability: Not all Chinese mills produce G3106 plates above 50mm with through-thickness properties
- Request ultrasonic testing (EN 10160 or JIS G0801) for thicknesses ≥ 20mm in critical welds
FAQ
Can SS400 substitute for SM400B in welded structures?
Only for non-critical, lightly loaded joints with low consequence of failure. For code-compliant construction (buildings, bridges, ships), substituting SS400 for SM400B risks rejection during inspection and voids insurance coverage.
Why is SM400C more expensive than SM400A?
Three factors: (1) tighter chemical control requires more selective scrap charging; (2) -20°C impact testing adds laboratory cost and rejection risk; (3) lower carbon limits reduce furnace yield by 2 – 3%.
Do Chinese mills hold JIS certification?
Approximately 15 – 20 Chinese plate mills maintain JIS Mark certification through JQA or BV audits. Many more produce to JIS chemical/mechanical requirements without formal certification — acceptable for some markets but not for Japan domestic projects requiring JIS Mark.
What thickness range is available for SM400 plates?
Standard production: 6 – 150mm thickness, 1,500 – 4,000mm width. Ultra-thick plates (150 – 300mm) require specialized TMCP mills — only 4 – 5 Chinese suppliers capable.
How do I verify authenticity of JIS MTC from China?
Cross-check mill logo, heat number format, and laboratory accreditation (CNAS Lxxxx). Contact the mill directly via published email (not phone numbers on MTC alone) to verify heat number existence. SGS or BV inspection during production provides highest confidence.
Procuring JIS-certified structural steel from China? Huaxia-Steel supplies SS400 and SM400A/B/C with full JIS compliance documentation, third-party inspection, and custom plate cutting.
